Create Partition i nexisting table

Hi,
We have sql serer 2005 and database is very big and some of the tables are very big.
We have three Database - SearchDb, MSDB1, MSDB2
SearchDB has table called - KeyTab and it has PK field called ID which has Identity and another column called AU_ID which is served as
Key Field to join another table and another table has same field.
We are planning to add new column called Group ID and make it as Partition Column, this group id will consist in a three Range/group
1, 2, 3 which will we our partition.
Once i add the column Group Id into table. I need to create the partition into this existing table.
1) I will add the three file group into existing table using following:
2) Create the partition Function
3) Create the Partition Scheme
Now i need to create the procedure to Insert the data into different Partition using following criteria:
Check the AU_ID into SearchDB.KeyTab and compare with MSDB1.AUSUMTab and
if StartDate > today's Date then Insert into Partition 1 of SearchDB Database of Partitioned Table KeyTab
if StartDate < today's Date then Insert into Partition 2 of SearchDB Database of Partitioned Table KeyTab
and another logic for EndDate i need to add as
if EndDate < today's Date then Insert into Partition 3 of SearchDB Database of Partitioned Table KeyTab
Could you please guide me that my above steps are right and also how i can write the Procedure to Insert data into Partition?
Thanks,